They didn't early on, but I'm seeing it more and more now. Sometimes they even have "Rental" printed on the disc itself.
Oh, okay, that's what I thought. So my question is still unanswered.
One question that maybe can be answered, though: if one has a conventional, non-HD television set, the kind with a picture tube rather than a flat screen, would there be any visible advantage in watching a Blu-ray disc rather than a conventional DVD?
First screen technology has nothing to do with HD. I've seen CRT HDTVs and LCDs that are non-HD. But assuming you have a standard 640x480 resolution TV then you won't be able to see any benefit from Blu-ray.
Theoretically you can still here the uncompressed audio, but if you don't have an HDTV I'm going to assume you don't have decent speakers to hear the audio either.